A non-volatile memory device is capable of retaining stored information even when power to the memory device is turned off. Traditionally, non-volatile memory devices occupied large amounts of space and consumed large quantities of power. As a result, non-volatile memory devices have been mainly used in systems where limited power drain is tolerable and battery-life is not an issue.
One type of non-volatile memory device includes resistive memory cells as the memory elements therein. Resistive memory elements are those where resistance states can be programmably changed to represent two or more digital values (e.g., 1, 0). Resistive memory elements store data when a physical property of the memory elements is structurally or chemically changed in response to applied programming voltages, which in turn changes cell resistance. Examples of variable resistance memory devices include memory devices that include memory elements formed using, for example, variable resistance polymers, perovskite materials, doped amorphous silicon, phase-changing glasses, and doped chalcogenide glass, among others. Memory access devices, such as diodes, are used to access the data stored in these memory elements. FIG. 1 illustrates a general structure of a cross point type memory device. Memory cells are positioned between access lines 21, 22, for example word lines, and data/sense lines 11, 12, for example bit lines. Each memory cell typically includes a memory access device 31 electrically coupled to a memory element 41.
As in any type of memory, it is a goal in the industry to have as dense a memory array as possible; therefore, it is desirable to increase the number of memory cells in an array of a given chip area. In pursuing this goal, some memory arrays have been designed in multiple planes in three dimensions, stacking planes of memory cells above one another. However, formation of these three-dimensional structures can be very complicated and time consuming. One of the limiting factors in forming such three-dimensional memory structures is the formation of the memory access devices. Traditional methods may require several expensive and extra processing steps and may also cause damage to previously formed materials during formation of subsequent materials.
Therefore, improved fabrication methods for forming memory access devices are desired.